Because combining them slows the cycle time of the processor and the
designers are trying to maximize the weighted speed, namely the frequency
of occurance over all programs of each operation times the cost of that
operation. For any given workload a workload-specific design is always
better, but the speed over all workloads is worse.

As workloads change and and our understanding of them changes, processor
designs are optmized to those different workloads. The pessimizaton of
atypical workloads increases also.